It is known that engine filters have an inlet flow path for dirty fluid that needs to be filtered, and an outlet flow path for clean fluid that has been filtered by the filter media of the filter. The inlet and outlet flow paths are typically sealed from each other to prevent dirty fluid from entering the outlet flow path and mixing with the clean fluid which can degrade downstream components. In spin-on filters, the inlet and outlet flow paths are typically at the same end of the filter, and in many spin-on designs, the inlet and outlet flow paths are defined by a nutplate of the filter.
In some fuel filters, it is advantageous to pre-fill the dirty side of the filter with dirty fuel. To permit pre-filling, it is known to supply a cap that is used to close off the clean fuel outlet during pre-filling, with the cap then being discarded.